The invention relates to a method and apparatus for monitoring a patient for rejection reactions to an implanted organ, especially to an implanted heart.
Rejection of the transplanted organ, especially of a transplanted heart, represents a substantial problem in transplant medicine. Thus it is important to diagnose an incipient rejection reaction as early as possible, in order to be able to institute timely immunosuppressive therapy.
In addition to invasive endomyocardial biopsy and noninvasive echocardiography, an electrophysiological diagnostic procedure is also known in which the intramyocardial electrogram is measured by means of implanted electrodes. Preferably an implantable telemetry pacemaker, which transmits the measured values via an inductive coupling to an extracorporeal instruments (such as disclosed in U.S. Pat. Nos. 4,809,697; 4,585,004; 4,237,900; 4,550,370; 4,281,664), is used as the measuring unit in this procedure. This diagnostic procedure permits continuous monitoring of the patient, by the fact that the electrogram is measured and checked at short time intervals. When data telecommunication is employed, moreover, the patient can also be monitored without having to visit a clinic for that purpose.
The measurement and monitoring of the intramyocardial electrogram is subject to some uncertainties, since the measured results can also be modified by other influences, such as the variations of daily rhythm in the electrocardiogram, the exercise status of the patient and the medication. Thus a drop of the measured cardiac voltage signals is not necessarily due in each case to an incipient rejection reaction.
The object of the invention is to provide, for monitoring a patient for rejection reactions to an implanted organ, especially a transplanted heart, a method and apparatus that represents an addition to the known methods and that improves the diagnosis.
The invention is based on the knowledge that functional changes of organs and especially functional changes of the heart are associated with changes of the electrophysiological properties of the tissue, which changes can be detected by a change of the electrical resistance and especially the impedance of the tissue (B. C. Penney et al., Medical & Biological Engineering & Computing, 1985, 23, p. 1-7). It is assumed that these changes are due to interstitial edemas, infiltration in the interstitial tissues and electrophysiological changes of the cell membranes and of their capacitive properties.